I have a vanilla server hosted on my PC for 1.16.1. Everything on it works perfectly fine, and has for a long time now. However, I've just tried to put a custom resourcepack on there, and no matter where I upload it, it does not automatically download when you join the server. I have 'enabled' server resourcepack under minecraft itself.
The download link (www.dropbox.com/s/b6buk0p1moyabst/US6.zip?dl\=1) [If it doesn't work, copy it directly] is a direct link. If you click that, it will download the resource pack. I've tried it with both https and without (as some people said that can muck it up), and it doesn't work.
The only issues I can think of are 1. Something is wrong with the resource pack itself (but I can't find anything) that stops it from being recognised as a resource pack, or 2. something wrong with the server client that doesn't recognise the resource pack, no matter what.